Commercial, or civil, aircraft are conventionally designed for reducing exhaust emissions from combustion of hydrocarbon fuel such as, for example, Jet-A fuel. The exhaust emissions may include hydrocarbon particulate matter, in the form of smoke, for example, carbon monoxide, and nitrogen oxide (NO.sub.x) such as, for example, nitrogen dioxide NO.sub.2. NO.sub.x emissions are known to occur from combustion at relatively high temperatures, for example over 3,000.degree. F. (1648.degree. C). These temperatures occur when fuel is burned at fuel/air ratios at or near stoichiometric, or, alternatively, at or near an equivalence ratio of 1.0, which represents actual fuel-air ratio divided by the stoichiometric fuel-air ratio. The amount of emissions formed is directly related to the time, i.e. residence time, that combustion takes place at these conditions.
Conventional gas turbine engine combustors for use in an engine for powering an aircraft are conventionally sized and configured for obtaining varying fuel/air ratios during the varying power output requirements of the engine such as, for example, lightoff, idle, takeoff, and cruise modes of operation of the engine in the aircraft. At relatively low power modes, such as at lightoff and idle, a relatively rich fuel/air ratio is desired for initiating combustion and maintaining stability of the combustion. At relatively high power modes, such as for example cruise operation of the engine in the aircraft, a relatively lean fuel/air ratio is desired for obtaining reduced exhaust emissions.
In the cruise mode, for example, where an aircraft gas turbine engine operates for a substantial amount of time, conventional combustors are typically sized for obtaining combustion at generally stoichiometric fuel/air ratios in the dome region, which represents theoretically complete combustion. However, in practical applications, exhaust emissions nevertheless occur, and conventional combustors utilize various means for reducing exhaust emissions.
Furthermore, it is generally desirable to have combustors which are as short as possible for reducing the overall weight of the combustor and gas turbine engine, as well as reducing parasitic cooling air requirements thereof. However, combustion gases discharged from the combustor must be provided to a conventional turbine disposed downstream thereof at relatively uniform temperature without undesirable hot streaks which would adversely affect the life of the turbine. Relatively uniform combustion gas exit temperatures are typically obtained by providing dilution air which is mixed with the hot combustion gases and undergoes mixing over a finite length of the combustor. Accordingly, the combustor must be sufficiently long for allowing such mixing to take place for obtaining relatively uniform temperatures.